U.S. Pat. No. 6,264,650 (“Hovda”) discloses methods for therapeutically applying electrical energy to tissue within a patient's spine, including introducing an active electrode into the patient's spine, positioning an active electrode near a target tissue, and applying a high voltage across the electrode to produce a plasma to volumetrically remove or ablate the target tissue.
Hovda is primarily directed to ablation techniques, such as laminectomy/discectomy procedures, wherein problematic tissue is removed or ablated. However, Hovda also discloses methods for shrinking collagen tissue. In particular, Hovda discloses treating fissure or tears within the inner wall of the annulus fibrosus of a degenerative disc by applying a voltage across the electrodes to heat the fissure and shrink the collagen fibers, thereby creating a seal or weld within the inner wall of the annulus fibrosus. See Hovda at col. 10, line 64.
In some embodiments, Hovda discloses delivering a conductive fluid to the target site to substantially surround the active electrode with a conductive media. Hovda teaches that this conductive fluid should have a threshold conductivity to provide a suitable conductive path between the return electrode and the electrode terminal. Although Hovda repetitively discloses the use of isotonic or normal saline as the conductive fluid, Hovda also teaches that a more conductive fluid, or one with a higher ionic concentration, will usually provide a more aggressive ablation rate, and reports that a saline solution with higher levels of sodium chloride than conventional saline (which is on the order of about 0.9% sodium chloride), e.g., on the order of greater than 1% or between about 3% and about 20%, may be desirable. See Hovda at col. 19, line 31.
In most heating embodiments in Hovda, Hovda teaches heating the target tissue by passing the radiofrequency (“RF”) current through the target tissue, wherein the tissue's resistance to the current produces the desired heating. However, in one embodiment, Hovda further teaches first passing the RF current through a conductive fluid to resistively heat the conductive fluid, and then directing the heated conductive fluid to the target tissue. In this embodiment, Hovda teaches that the primary mechanism for imparting energy to the tissue is the heated fluid, rather than the electric current. See Hovda at col. 13, line 28, and col. 39, line 14.
Hovda teaches many methods of changing the depth of the heating within a tissue, including a) changing the power level of the apparatus, b) adding a resistor to the device, c) adding a voltage reducing element to the device, d) changing the frequency of the RF current, and e) changing the electrode diameter.
In one embodiment, Hovda discloses increasing the temperature of the target tissue to 60° C. at a depth of 1-5 mm by either direct heating (i.e., deep penetration of current) or by heating indirectly (exposure to an RF heated fluid). See Hovda at col. 13, line 32. Hovda repeatedly teaches that the depth of penetration of the thermal damage is typically less than 5 mm. See Hovda at col. 10, line 47; col. 13, line 28; col. 30, line 47; and col. 38, line 25.
In one embodiment, Hovda teaches that the conductive fluid between the active and return electrodes will generally minimize current flow into the surrounding tissue. Thereby minimizing thermal damage to the tissue, so that severed blood vessels on the surface of the hole may not be coagulated as the electrodes advance through the tissue. Therefore, in this embodiment, Hovda teaches that the presence of the conductive fluid also essentially acts as an electrical short that keeps the surrounding tissue cool. See Hovda at col. 16, line 21.
Hovda further teaches that low impedance pathways provide low heating levels. See Hovda at col. 17, line 37.
In sum, Hovda discloses using isotonic saline for both ablation and more mild electrosurgical techniques such as tissue coagulation and teaches the use of hypertonic saline only for more severe ablation procedures.
U.S. Pat. No. 6,099,514 (“Oratec”) discloses a method of treating interverterbal discs by resistively heating. Oratec discloses the use of normal or isotonic saline.
U.S. Pat. No. 6,015,406 (“Goble”) discloses an electrode-containing electrosurgical instrument for treating tissue in the presence of an electrically-conductive fluid (“ECF”) medium. Goble discloses only normal saline as the electrically-conductive fluid medium.
US Pat. No. 5,433,739 (“Sluitjer”) discloses a method of therapeutically treating the disc by heating the inside of the disc with RF energy, microwave current, resistive heating and heating by ferromagnetic seeds. Sluijter characterizes the disc as “consisting of a fibrous structure with a substantially low electrical impedence.”
Goldberg et al., Radiology April 2001, 219(1), pp. 157-165, (“Goldberg”) reports on the effects of NaCl concentration on tissue conductivity, radiofrequency (RF) deposition, and heating in phantom models, and further teaches optimization of an adjunctive NaCl solution injection for RF ablation in an in vivo model. In the phantom models, Goldberg reports that NaCl concentration has significant but non-linear effects on electrical conductivity and RF deposition, and observed progressively greater heating up to 5.0% NaCl, with reduced temperatures at higher concentrations. In the in vivo studies upon normal well-perfused liver, maximum coagulation (7.0 cm) was observed to occur with injection of small amounts of saturated (38%) NaCl solution. Goldberg concluded that injection of NaCl solution before RF ablation can increase energy deposition, tissue heating, and induced coagulation, which will likely benefit clinical RF ablation.
However, Goldberg also reported the existence of non-uniform NaCl concentration in some of the models. Since Goldberg is concerned with tumor ablation, Goldberg found this uncontrolled non-uniform concentration to be an undesirable feature. Therefore, Goldberg suggests that one drawback to using hypertonic NaCl levels was a possible inability to control the uniformity of its distribution.
In addition, Goldberg reported that HCF can significantly migrate within collagen-based tissue to the point where it crossed a tissue boundary from the liver to the gall bladder. Therefore, Goldberg suggests that another drawback to using hypertonic NaCl levels was a possible inability to control its migration.
In sum, Goldberg was concerned mostly with eradicating tumors, and found that “the extent of tissue heating and coagulation were significantly influenced by both the volume and concentration of the NaCl solution injected.” However, Goldberg further cited concerns as to the possible inability to control the distribution and migration of the injected HCF. Goldberg concluded that “it will be important to determine whether the strategy of injecting NaCl solutions can alter tissue coagulation in a predictable and reproducible fashion so that the volume of coagulation induced can be appropriately matched to tumor size and location.